Mark Brazil, 23, of Lloyd Road, Chichester, was in a park with Benjamin Taylor-Baker.
Brazil and Taylor-Baker approached the victim, then 16, who was with friends, separated him from the group and then asked to borrow his ring, which belonged to his late grandfather and was valued at about £500.
The teenager refused to hand it over, and Brazil then hit him and tripped him over.
The boy managed to make it back to his friends, but Brazil grabbed him by the throat and threatened to hurt him, saying: ‘Give me your ring or I’ll cut your finger off.’
Due to fear of violence, the boy handed the ring over.
Taylor-Baker, 26, of Francis Close, Haywards Heath, then demanded the victim remove his clothes and shoes.
Both defendants then ordered the victim’s friend – aged 17 – to punch him in the face several times, which he also did out of fear of violence. The pair stated that if he refused, they would stab the victim.
The men then left the park but were later identified and arrested.
It happened in Clair Park in Haywards Heath on October 15, 2020.
Brazil and Taylor-Baker were charged with robbery and common assault, and the case went for trial.
After a number of delays due to coronavirus, a jury returned unanimous guilty verdicts at Lewes Crown Court.
Brazil was sentenced to seven years’ imprisonment and given an eight-year restraining order.
Taylor-Baker was sentenced to six years’ imprisonment and also given an eight-year restraining order.
Investigating officer Lauren Chapman, of West Sussex CID, has praised the bravery of the victims in seeing the case come to court.
She said: ‘This was a truly humiliating incident for the victim and his friends, who were forced to follow orders from Brazil and Taylor-Baker out of fear of violence.
‘It was a completely unprovoked attack on a group of innocent young people who were simply minding their own business, and it goes far beyond bullying.
‘I’d like to commend the bravery of the victim – who was subjected to physical and emotional abuse by two senseless yobs – and his friends. By coming forward and assisting us with our investigation, they have helped us to secure significant sentences in court.
‘The ring was of huge sentimental value to the victim, being an heirloom piece, and it was the first time he had worn it in public. It is a loss which is far greater than the cost value.
‘This was a long-awaited trial, and the unanimous guilty verdicts demonstrates the strength of the evidence. The victims received specialist support and were granted special measures, meaning the trial was accessible to them, and I hope this outcome just goes to show that even when there are delays well out of police control, we will keep fighting for justice for our victims.’